November 22, 2010
(Photo: Max Brown/STOCK.XCHNG)
Like musical compression saves space on
your mp3 player, the human brain has ways of recoding sounds to save
precious processing power.
To whittle a recording of your favorite song down to a manageable
pile of megabytes, computers take advantage of reliable qualities of
sounds to reduce the amount of information needed.
Collections of neurons have their own
ways to efficiently encode sound properties that are predictable.
"In perception, whether visual or
auditory, sensory input has a lot of structure to it," said
Keith Kluender, a psychology professor at the
University of Wisconsin-Madison.
"Your brain takes advantage of the
fact that the world is predictable, and pays less attention to
parts it can predict."
Along with graduate student Christian
Stilp and assistant professor Timothy Rogers, Kluender
co-authored a study published in this week's (Nov. 22) early online
edition of the Proceedings of the National Academy of Sciences
showing listeners can become effectively deaf to sounds that do not
conform to their brains' expectations.
The researchers crafted an orderly set of novel sounds that combined
elements of a tenor saxophone and a French horn. The sounds also
varied systematically in onset - from abrupt, like the pluck of a
violin string, to gradual, like a bowed string.
These sounds were played in the
background while test subjects played with
After a little more than seven minutes, listeners completed trials
where they were asked to identify one sound in a set of three that
was unlike the other two.
Distinguishing sounds that varied in instrument and onset in the
same way they had just heard was a simple matter. But sounds that
didn't fit - with, say, more pluck and not enough saxophone - were
completely lost to the listeners.
They could not correctly identify one of
the non-conforming sounds as the odd one among three examples.
"They're so good at perceiving the
correlations between the orderly sounds, that's all they hear,"
says Kluender, whose work is funded by the National Institute of
Deafness and Other Communication Disorders.
"Perceptually, they've discarded the
physical attributes of the sounds."
The results jibe well with theoretical
descriptions of an efficient brain, and the researchers were
able to accurately predict listener performance using a
computational model simulating brain connections.
"The world around us isn't random,"
"If you have an efficient system,
you should take advantage of that in the way you perceive the
world around you. That's never been demonstrated this clearly
To avoid having to carefully take in and
remember every last bit of visual or audible stimulus it encounters,
the mind quickly acquaints itself with the world's predictability
"That's part of why people can
understand speech even in really terrible conditions," Kluender
"You can press your ear to the wall
in a cheap apartment and make out a conversation going on next
door even though the wall removes two-thirds of the acoustic
information. From just small pieces of sounds, your brain can
predict the rest."